It is routine for gas turbine engines to have to pass stringent vibration acceptance tests following production. Rotor eccentricities are a main source of engine vibration, and eccentricities can be alleviated by rotor balancing. Examples of how rotors are balanced without compromising their structural integrity include oversizing a part of the rotor disk then removing material in selected portions of that part, and providing an additional removable ring from which material is removed in selected portions thereof. However these approaches generally increase the weight of the rotor.